An interior permanent magnet motor (hereinafter called an IPM motor) including a permanent magnet embedded inside a rotor can generate reluctance torque as well as magnet torque resulting from attractive force/repulsive force between a coil and the permanent magnet, and therefore compared with a surface permanent magnet motor (SPM motor) including a permanent magnet attached to an outer periphery of a rotor, the IPM motor has higher torque and higher efficiency. Therefore such an IPM motor is used as driving motors in hybrid vehicles, electric vehicles and the like requiring high output performance. Typically used permanent magnets therefore include sintered magnets of rare-earth magnets, ferrite magnets, alnico magnets and the like.
In order to realize smooth insertion of a permanent magnet into a slot formed in a rotor core and to avoid damage of the permanent magnet at a slot edge, in general, the IPM motor is designed so that the slot is horizontally longer and has a larger dimension than the permanent magnet, and a space defined by a lateral side face of the magnet and a slot face is filled with resin of a non-magnetic material, followed by curing of the resin, thus fixing the permanent magnet.
Referring to FIG. 8, such a state of a magnet fixed inside a slot is described below.
FIG. 8a partially shows a stator S provided with a coil C around a tooth T and a rotor R including permanent magnets PM in a predetermined number embedded therein, the rotor R being arranged rotatably inside the stator S to make up a conventional IPM motor.
At a rotor core making up the rotor R is bored a rotor slot RS to contain the permanent magnets PM, and a lateral side of the rotor slot is filled with non-magnetic resin F1, F2 to fix the permanent magnet PM. In the illustrated example, two permanent magnets PM are arranged like a substantially V-letter shape to form one magnetic pole.
Meanwhile, the resin F1, F2 should naturally fix the permanent magnets PM from the lateral sides thereof in the rotor slots RS, and further has a function as a flux barrier to suppress flux leakage from the permanent magnets PM. As a shape to suppress the flux leakage MJ from the permanent magnets PM, the resin F1, F2 has a shape as shown in FIGS. 8a and 8b, for example.
Herein, as is easily understood, the flow of flux J from the stator side into a permanent magnet PM provided in the rotor tends to pass through the rotor core having high magnetic permeability, and therefore the flux J entering from the stator side tends to concentrate on a corner area of the permanent magnet PM on the stator side.
Referring to FIG. 8b, such a tendency is described below. The resin F1, F2 on the lateral sides of a permanent magnet PM have thicknesses t1′ and t1″ at a part in contact with the permanent magnet PM that are smaller than the thickness t1 of the permanent magnet PM. That is, since the slot is bored in this area to have dimensions and shapes for resin F1, F2 having the thicknesses t1′ and t1″ smaller than the thickness t1, the permanent magnet PM can be aligned at their lateral edges K1 and K2.
If the resin F1, F2 has a thickness at a part in contact with the permanent magnet PM larger than the thickness of the permanent magnet PM, then the permanent magnet PM cannot be aligned in the slot, which may influence on magnetic characteristics of the motor.
In this way, the permanent magnet PM can be securely aligned in a sophisticated manner at the lateral edges K1 and K2. However, since the resin F1, F2 having such dimensions and shapes is formed on the lateral sides of the permanent magnet PM, the resin F1, F2 will have parts therein with thicknesses t2 and t3 that are significantly smaller than the thickness t1 of the permanent magnet PM.
Then, as stated above, since the flux J from the stator tends to pass through the rotor core having high magnetic permeability, the flux J likely passes through not the permanent magnetic PM having the thickness t1 but the routes through the thicknesses t2 and t3 in the resin F1, F2 that are the shortest routes to be reachable to the rotor core having high magnetic permeability. Then, during the course of passing through these routes, the flux J will concentrate on and pass through the corner areas of the permanent magnet PM on the stator side, thus increasing demagnetizing field that acts on the corner area of the permanent magnet PM on the stator side.
Herein the demagnetizing field is made up of the sum of internal magnetic field flowing from N pole to S pole inside the magnet and the above-stated external magnetic field entering from the stator side, among which the external side can be said to mainly decide the magnitude and the direction of the demagnetizing field.
In order to secure a desired coercive force against this demagnetizing field, there is a need to increase a coercive force of the magnet, especially at the corner areas on the stator side.
Then, as typical measure to improve this coercive force of a permanent magnet, the alloy composition making up the permanent magnet is partially replaced with Dy (dysprosium) or Tb (terbium) that are metals having high coercive force performance so as to increase anisotropy field of the metal compound and so increase the coercive force. However, the usage amount of dysprosium or terbium greatly exceeds the natural abundance ratio of rare-earth elements and additionally the estimated amount of commercially developed deposits is extremely low, and moreover the existing regions of the deposits are eccentrically located across the world, and therefore the necessity of strategy for these elements has been recognized.
Even if a coercive-force distribution magnet is manufactured including dysprosium or the like with the impregnation amount corresponding to a required coercive force varying with each part of a magnet, in order to give a coercive force to the magnet against high demagnetizing field, more dysprosium or the like has to used for the part, resulting in the failure to effectively reduce the usage amount of dysprosium or the like.
In view of such a present situation, the present inventors came up with the idea of a rotor capable of reducing demagnetizing field that might be generated in a magnet by modifying the shapes and the structures of both of a slot bored in the rotor and a magnet fixed in the slot, and accordingly capable of reducing a coercive force required for the magnet, and so reducing the usage amount of expensive rare metals used to increase coercive force performance of the magnet.
Conventionally Patent Document 1 discloses a technique for a rotor including a flux barrier in a L-letter shape in planar view formed at a corner of a slot.
This rotor is provided with the flux barrier in a L-letter shape in planar view at a corner of the slot, whereby cogging torque of the motor can be reduced. However, even in this configuration, the flux from the stator side as shown in FIG. 8b still tends to pass through this L-shaped flux barrier because this flux barrier also has a smaller thickness than the thickness of the permanent magnet. Therefore, demagnetizing field that might be generated at a corner area of the permanent magnet on the stator side still remains high, and it is still difficult to reduce the usage amount of dysprosium or the like used to secure a coercive force against this demagnetizing field.    Patent Document 1: JP Patent Publication (Kokai) No. 2000-278896 A